Taxonomy & naming
Serrasalmus nigricauda was described by the German-Argentine naturalist Hermann Burmeister in 1861, based on material from the Río Paraná system in Argentina, which remains the species' documented type locality. It belongs to the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes, and to the large genus Serrasalmus, whose many species are notoriously difficult to distinguish from one another and have a long history of taxonomic revision.
The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the currently valid name and combination; this account follows that placement in Serrasalmus. As with many members of the genus, field and aquarium identification of S. nigricauda relies on a combination of body proportions, dentition and colour pattern, and it can be confused with related Paraná-basin Serrasalmus species without close comparison.
Morphology
Serrasalmus nigricauda has the classic piranha build shared across the genus: a deep, laterally compressed body, a blunt, undershot lower jaw, and a single row of interlocking, triangular, razor-edged teeth in each jaw that shear rather than crush. A FishBase-derived length-weight relationship (Bayesian estimate a = 0.01380, b = 3.16, total length) has been published for the species, but no confirmed maximum published length is available in the sources consulted for this article, so a specific size figure is left as data sparse here rather than guessed.
Colour pattern and fin markings in Serrasalmus generally track close relatives in the Paraná basin, but detailed, species-specific descriptions of adult colouration were not found in the sources available for this article. Prospective keepers and researchers should treat photographs sold under this name with some caution given how frequently Serrasalmus species are mislabelled in the trade.
Habitat
Serrasalmus nigricauda is documented from the Río Paraná system in Argentina, part of the broader Paraguay-Paraná basin that drains a large portion of south-central South America. FishBase classifies it as a freshwater, benthopelagic species, consistent with the river-channel and associated habitat typical of Serrasalmus piranhas in this basin.
Detailed water-chemistry data specific to this species were not located in the sources consulted, though related Serrasalmus species in the literature (S. compressus, S. rhombeus, S. maculatus) are documented in roughly 73–82 °F water at pH 5.5–7.5, and the subtropical latitude of the Paraná system means conditions there can run cooler seasonally than in equatorial Amazonian habitats. This should be treated as a genus-level guide rather than a confirmed reading for S. nigricauda specifically.
Feeding
FishBase describes Serrasalmus nigricauda as an opportunistic carnivore that feeds on fins and pieces of other fish, consistent with the fin-and-flesh-nipping behaviour documented across the genus rather than the coordinated pack-hunting of documentary lore. FishBase also notes the species has high resilience and a low vulnerability to fishing pressure, both indicators of an adaptable, generalist feeding strategy.
In the aquarium, related Serrasalmus species are kept on a varied diet of meaty foods — fish fillet, prawn and mussel — and can often be weaned onto dead foods rather than live prey; the same approach is the sensible default here. Live feeder fish should be avoided on general welfare and disease-risk grounds rather than fed as a matter of routine.
Mating
No species-specific courtship description for Serrasalmus nigricauda was located in the sources consulted. Piranha-guide literature covering the genus broadly describes Serrasalmus as solitary and territorial outside of pairing, in clear contrast to the shoaling Pygocentrus, meaning a compatible pair typically needs to be introduced deliberately into a large, well-structured tank rather than allowed to sort itself out of a group.
External sexual dimorphism is not documented for this species in the sources available; as with most Serrasalmus, sexing is likely to rely on behaviour and condition rather than reliable external markers.
Breeding
General piranha-keeping literature describes Serrasalmus as substrate egg-layers: a pair clears and defends a nest site among plants or root structure, the female releases adhesive eggs that are fertilised and guarded, with fanning most often attributed to the male, and the eggs hatch within a few days. This is the genus-typical pattern and is presented here as the expected mode for S. nigricauda; no confirmation of successful captive breeding for this specific species was found in the sources consulted, so keepers should treat it as effectively unbred in the hobby rather than a routinely spawned aquarium fish.
It is not a mouthbrooder or bubble-nest builder, and its reproduction should not be confused with the very different long-distance migratory spawning of pacus or the flannelmouth characins.
In the aquarium
Serrasalmus nigricauda should be treated the way the genus generally is: a solitary, territorial predator rather than a shoaling fish, and kept singly in a large aquarium with dense cover from driftwood and robust plants rather than crowded with conspecifics, which tend to fight. Given the lack of confirmed size data for this species, keepers should plan for a substantial tank as a precaution and expect a fish on the order of other mid-sized Serrasalmus rather than something that will stay small.
Diet should be varied meaty food rather than exclusively live fish, and the powerful jaws and teeth mean the fish should always be moved by container, never by hand or through mesh. As with all piranhas, local and state law should be checked before acquisition — Serrasalmus species are restricted or banned in many U.S. states and other jurisdictions — and a piranha must never be released into open water.
Conservation
Serrasalmus nigricauda has not been evaluated by the IUCN Red List, so its conservation status is Not Evaluated rather than a confirmed category. FishBase's assessment of high resilience, a short minimum population-doubling time and low vulnerability to fishing all point toward a species that is not inherently fragile, but that is not a substitute for a formal assessment.
No specific threats or population trends for this species were located in the sources consulted; general pressures on Paraná-basin freshwater fish — damming, water-quality change and fishing pressure across the broader system — are relevant background but should not be presented as documented findings specific to S. nigricauda.